For efficient use of solar energy, many developments are being conducted in technologies to convert the solar energy into a form which can be easily used and then store the same. One typical example thereof is a solar battery. However, the solar battery has not spread because of high cost thereof, and there is a demand for development of a cheaper system.
Accordingly, studies are being conducted in technologies to convert light energy to chemical energy using photocatalysts. When being irradiated by solar energy, the photocatalysts absorb the light energy to generate electrons and holes and cause a chemical reaction. Among the photocatalysts, especially in titanium dioxide (TiO2), the valence band is located deep, and oxidizing power of generated holes thereof is stronger than that of chlorine or ozone. Accordingly, a method of splitting water using a semiconductor photoelectrode made of TiO2 has been studied (see A. Fujishima and K. Honda, Nature, 238(5358), 37 (1972)).
For the conventional semiconductor photoelectrodes, TiO2 single crystals or TiO2 sintered pellets obtained by sintering and pelletizing TiO2 powder were used. However, use of the TiO2 single-crystals increased the manufacturing cost, and use of the TiO2 sintered pellets reduced the photoelectric conversion efficiency.
In recent years, therefore, a semiconductor photoelectrode was disclosed in which a semiconductor layer was formed on a substrate composed of conductive glass or smooth metal (see J. Phys. Chem., 102 (1998) 7820 and J. Phys. Chem., 98 (1994) 5552). However, use of the conductive glass for the substrate increased the cost and moreover might degrade adhesiveness and stability of the interface between the substrate and semiconductor layer. Accordingly, a semiconductor photoelectrode including a metallic substrate as the substrate is being developed.
FIG. 11 shows a semiconductor photoelectrode 40 including a titanium plate (Ti plate) as the substrate. In the semiconductor photoelectrode 40, a Ti plate is baked to form a TiO2 layer on the Ti plate, thus forming a semiconductor layer (TiO2 layer) 42 on a substrate 41. Baking the Ti plate to form the semiconductor layer (TiO2 layer) 42 on the surface thereof in such a manner reduces the manufacturing cost. Moreover, there is an advantage of obtaining a flexible semiconductor photoelectrode.